They rarely exist: that is to say, what’s happening lower on the ticket generally won’t have a great effect on the presidential race. The race this year could be close enough that such an effect could happen. In Florida, the Bush campaign desperately wants Mel Martinez to win the Republican Senate primary because it thinks it will get a boost in the state if he is on the ticket. (I think the campaign is right about that.) In Missouri, Republicans are hoping that the incumbent Democratic governor, Tim Holden, holds on in today’s primary. If he does, Republican Matt Blunt is favored to beat him–which could help Bush. (Of course, the party wants to win the governor’s race for its own sake, too.)